Guest post by David Flood: Height and herencia in rural Guatemala

Peter Rohloff has training in parasitology, internal medicine and pediatrics. He practices in Boston, MA at Brigham and Women's Hospital and Boston Children's Hospital. Since 2003, Peter has been working in Guatemala, where he serves as the medical director for a health systems NGO – Wuqu’ Kawoq | Maya Health Alliance. Peter’s interests include the management of chronic diseases of children and adults in resource poor settings and how indigeneity, cultural, and language barriers impact access to and utilization of health care.
@wuqukawoq

Over the past year, I have been working with the NGO Wuqu’ Kawoq on programs to prevent and treat chronic malnutrition in several indigenous communities in rural Guatemala. Chronic malnutrition, also known as “stunting,” is the result of a complex social and biological interplay and manifests in children who are very short for their age. Children who are short grow up into adults who are not only short, but also who tend not to reach their academic, economic, and health potential.

It is hard to overstate the burden of chronic malnutrition in Guatemala. With a prevalence of stunting at approximately 50%, Guatemala has the highest rate of chronic malnutrition in the Western Hemisphere and the third highest rate in the world. Even among its Central American neighbors and income peers, Guatemala’s burden of chronic malnutrition is striking:

Moreover, the World Bank estimates that indigenous children are twice as likely to suffer from stunting as non-indigenous children in Guatemala. In the rural communities where Wuqu’ Kawoq works, we often encounter rates of stunting greater than 70%. In fact, in one of our poorer communities where we monitor the growth of over 200 children ages 6 months to 5 years, every single child in the community is below the WHO median height-for-age.

Thinking about the height of my patients on a daily basis has led me to a simple question: What is the relative importance of nature vs. nuture on the empiric reality that Maya indigenous children and adults in rural Guatemala are short? In other words, if one could remove the social and environmental forces that influence chronic malnutrition–deep poverty, food insecurity, lack of education, low access to quality health care services, inadequate land holdings–and look directly into the genes, what would be the average height of a cohort of Maya children and adults?

This is the question I will try to reflect upon in this post. Here are the some key points I’ve come away with in my own investigation of this topic:

1. It is difficult to find well-nourished indigenous populations to study.

I’m not the first person to wonder about the influence of nature vs. nuture on the height of indigenous people in Guatemala. This question also interested investigators in the famous Guatemalan INCAP trial. In a 2010 article reviewing 50 years of the INCAP trial, one INCAP researcher offered this shocking admission:

The question of what relative importance to give to racial ancestry and to the nutrition–infection complex as causes for the differences in attained growth among Central American populations has been of interest to INCAP through its history … An obvious approach would be to compare the growth of well-nourished children of the various racial groups among themselves and with the reference population. However, the problem of studying certain groups, such as the Mayan people of Guatemala, was and still is that it is difficult to find well-nourished indigenous populations [emphasis mine].

In other words, one reason we don’t know how tall Maya indigenous populations might be is that malnutrition is universal in their communities. That is, there is no affluent, well-nourished Guatemalan Mayan community from which to draw baseline data.

2. In explaining differences in height in young children, social class is much more important than ethnic background.

When I first arrived in rural Guatemala and accompanied Wuqu’ Kawoq’s staff on community visits to conduct height-and-weight campaigns, I was struck by the fact that childrens’ heights-for-age were clustered around 2-3 Z-scores below the median. At first, it was tempting to ascribe such consistent findings to ethnic variation, but I soon found the largebody of literature starting in the 1970s showing that, as one 2010 study states, “all young children have the potential to grow similarly, regardless of their ethnic group or place of birth, if they are in a healthy environment and receive adequate nutrition.”

Comparisons among preschool children, presumably well nourished but of different ethnic background, indicate that differences in height and weight are relatively small–3% for height and about 6% for weight. In contrast, differences between these children and those, often of similar ethnic and geographical background, who live in the poor, urban and rural regions of developing countries approach 12% in height and 30% in weight. Thus, differences in growth of preschool children associated with social class, are many times those which can be attributed to ethnic factors alone. Therefore, height and weight standards chosen to represent optimal preschool growth can be drawn from studies of well-to-do already published children, regardless of race or ethnicity, because any racial or ethnic effect on mean preschool growth is small compared with environmental effects [emphasis mine].

The figure below shows mean length-for-age measurements in the WHO Multicentre Growth Reference Study Group, which included healthy, affluent children under 24 months of age in Brazil, Ghana, India, Norway, Oman, and the U.S. The data from these cohorts were used to create the 2006 WHO reference growth charts. You can see that the length-for-age is basically identical in each of the groups.

Source: WHO Multicentre Growth Reference Study Group, as cited in this paper.

In other words, much evidence shows that the effect of environment is greater than the effect of genes in explaining why a given group of young children is short. Therefore, our organization should not rely on ethnic explanations for short stature when considering why Maya children are short in our communities.

3. Maya children who grow up in the United States are taller than Maya children in Guatemala.

In 2010, ABC conducted a feature on Wuqu’ Kawoq’s nutrition program. As part of this feature, our organization produced images of 9-year-old Maya children in a rural Guatemalan community and images of 9-year-old Maya American who have grown up in Florida. The two photographs are shown below. Note how much taller than U.S.-raised children appear compared with their Guatemalan counterparts.

The claim made in this photo is not just a trick of television. In a number of studies published since the early 1990s, including a 2002 paper in the American Journal of Human Biology, biological anthropologist Barry Bogin has documented the incredible “plasticity” of linear growth in Maya children. Using datasets from multiple Maya communities in the U.S., Bogin and his colleagues have shown that, on average, Maya American children aged 5-12 years are 11.5 cm taller than Maya children living in Guatemala. These differences are statistically significant despite the fact that, as Bogin notes, the families of Maya American children were relatively poor by U.S. socioeconomic standards (but did have access to clean drinking water, medical care, and supplementary feeding programs).

The top two lines show the average height-for-age of 2 U.S.-based cohorts of Maya children. The bottom line is a comparison group of Maya children in Guatemala. Source.

In other words, if you take Maya children from poor families out of their Guatemala communities and put them in the U.S. with access to minimal services, the children grow much taller.

4. Using archaelogical samples, researchers have found that the pre-conquest Maya were taller than the modern Maya.

These are complex studies and far from my area of expertise. However, both investigations report a post-conquest decline in the stature in indigenous people. According to Márquez and Ángel, the present-day Maya of Mexico are about 2 centimeters shorter than Classic (A.D. 250-900) and Postclassic (AD 900-1500s) Maya and roughly 4 centimeters shorter than Pre-Classic (before A.D. 250) Maya. Using different data sources, Bogin and Keep come to a similar conclusion.

Estimates from long-bone data suggest that modern Maya are shorter than the ancient Maya. Source: Height among Prehispanic Maya of the Yucatan Peninsula: A Reconsideration, in Bones of the Maya.

Discerning the cause of this reduction in height is complex. However, one hypothesis is that the social, economic, and biologic turmoil of the Spanish Conquest created the very environment that has hindered the growth potential of Maya people and made malnutrition ubiquitous in Maya communities. In other words, as Bogin and Keep summarize:

To the extent that changes in mean stature for human populations reflect the “material and moral condition of that society” (Tanner 1986), the decline in adult stature after the year AD 1500 is likely due to the effects of the Conquest on native Latin Americans.

Conclusion

Although I am not on expert on the genetics of height, my own reading of the literature has led me to the believe that nuture, rather than nature, is a much more powerful explanatory factor when thinking about the short stature of indigenous Maya children and adults. Although ethnic variation may play a role in the ultimate ideal height in this population, it is currently impossible to discern how much of a role given that modern Guatemalan indigenous communities suffer from very high rates of chronic malnutrition, poverty, and social exclusion.

One of my colleagues at Wuqu’ Kawoq frequently talks to indigenous mothers about these issues. Here is what she says:

[And people say that the child is not growing because of its genes, because their grandmother was short or their grandfather was short, therefore they inherited being short, but that is not true. We are all short because we did not receive good nutrition during pregnancy and when we were growing.]

This is the message we want to send to the families of our malnourished children. We know these communities are poor, so we make sure to offer this information without any kernal of blame. At the same time, we try to help families understand that it is not herencia (inheritance) that makes their children short but rather environmental, economic, and social factors.

[…] Animatedly browsing the pages of her new social studies textbook, Jessica discusses her passion for the subject. She loves learning about politics, anthropology, and history, particularly that of her Mayan ancestors. Jessica encourages her classmates to take pride in their culture and learn as much about it as possible, proclaiming “It is only through learning about what happened to our ancestors that we can avoid suffering like they have.” While Jessica loves history, her ultimate goal is to become a doctor to help special needs children and those who suffer from malnutrition. […]